SEA GIRT, N.J. — “I have a time share with the dance floor — I own the front and center,” Chris Fedroff, 27, said to his friend in the basement level of the Parker House, a green and white Victorian manse here that is by day a sedate lunch spot, by night a throbbing epicenter of summer parties.

It was shortly after 5 p.m. on a Friday night in late July. Waiting with anticipation for the empty room called “the Tavern” to fill with strangers, Mr. Fedroff, a regular, was drinking a vodka soda and pacing the dimly lit room. The bartenders had just finished cutting lemons and limes. They had wiped down the bars that would soon be stained with drinks.

Upstairs on the restaurant level, parents with babies and women in their 80s were sitting on the quaint wraparound porch eating an early dinner — the house wedge and lobster are popular — and taking in a peaceful view of trees and sky. Teenagers rode their bikes in the middle of the street as police officers put cones out in the street.

Debauchery was in the air. The tide in this beach town was changing.

“This place is about to become crazy,” a woman said to the couple sitting next her at a nearby table. “Just you watch.”

Hundreds of 20-somethings who left work early to be here would arrive in taxis to pay a $10 cover and drink $2 beers until 9 p.m., when the special ends. They’d walk up the gravel driveway, get a stamp of invisible ink and sip quickly while they mingled and danced until the bar closed at 11:30. As the sun continued to go down, the energy only went up and the line became longer.

They come for more than just cheap drinks. They come to feel like they’re back in high school at the coolest kid’s house on a Friday night having a total rager that could potentially be broken up any minute.

“It’s like being at a house party,” said Barbara Onufrak, 23, who was waiting for a friend by the entrance. “You feel like someone’s hosting it and that you’re not at a bar. It’s so homey.”

Ms. Onufrak’s bank card was recently stolen, but she rushed to get a temporary one because she knew she’d be coming here and the Parker House, open since 1878, doesn’t accept credit cards.

“I took out $40, but I probably won’t use it tonight,” she said, laughing, knowing that it won’t be hard to find someone to buy her a drink.

The Parker House can quickly become a drunk person’s worst nightmare if one loses a friend. One block from the beach, the establishment is 20,000 square feet, spanning across three floors and the basement. Cellphone service is hit or miss. One room leads to another. It’s hard to navigate even sober.

Above the basement is the main floor with the restaurant and an indoor and outdoor area. There are 10 bars and 23 televisions. On any given Friday or Saturday night, 17 bartenders are serving on average 800 people or more, according to management.

On the top two floors are 32 rooms available to rent from Memorial Day to Labor Day for $3,800 to $4,200. All but six share a bathroom. This visit found a vacuum, containers of paint and a ladder scattered throughout the hallway. From a room upstairs, even with the door closed, the music could be felt and heard from below.

“I’ve heard a lot goes on up there,” said a young man at the bottom of a stairwell. “But I’d rather stay at my house down the block on the beach.”

Anyway, there was plenty going on below. People were doing shots, others were playing shuffleboard, and others beginning to dance. Every room played different music, every room had a slightly different crowd.

It was 6 p.m., and happy hour was already starting to get to people’s heads. A woman in a bathing suit and flip-flops stood next to a stranger in stilettos. A large group of guys there for a bachelor party, half of them wearing Hawaiian shirts, were beginning to drink in earnest. In a couple of hours they would be down in the basement dancing and double-fisting beers.

While some had come from New York City and others from nearby towns in New Jersey, most agreed on one thing: This place is the nothing like the Hamptons.

“You choose one or the other,” said Kaitlin Rabinowitz, a 24-year-old who lives in Manhattan, inside at one of the bars. “I would never go to the Hamptons.”

Her male friend, quiet until now, interjected.

“You’ve never even been,” he said. “You’d go if someone flew you there on a helicopter.”

Ms. Rabinowitz insisted that the Parker House, which (inevitably) has its own Instagram account with over 22,000 followers, “is like a fashion show” on Saturday nights and is where you want to be.

“This is the spot. They play ’90s music like Blink 182, and it takes me back to my childhood,” she said, while sipping the establishment’s famous cocktail named after a nearby town: the Spring Laker, made with Red Bull, vodka and pineapple juice.

“Everyone here is chill and laid-back,” Ms. Rabinowitz said. “Everyone is down to have a good time.”

The line to get in was starting to zigzag. There were last-minute makeup touches and friends who hadn’t seen each other since last weekend leaning across for a kiss on the check and giggling. Some had to get out of line to rush to the A.T.M. — conventionally placed right in the driveway — when they realized were a few dollars short for the cover.

Next to the long line was one just for “V.I.P.” cardholders. To land the card — which is looked upon as the American Express “Black Card” of Sea Girt — means skipping the line with a friend and the cardholder not paying a cover.

One woman rushed through, squealing with excitement, like she’d never been here before. She contemplated going down to the basement or straight ahead to the main floor. The fate of her night depended on her first move.

“I usually go down, but I don’t know,” she said to her friend. “Tell me what we should do.”

Down they went.

Some sang along to the lyrics, “now you’re just somebody that I used to know,” suspecting that the guy or girl they were dancing with — and in any moment kissing — would end up being somebody that they just knew for the night.

By 7 p.m., toilet paper was crumpled up on the bathroom floor, where women were reapplying lipstick and trying to stay within the lines.

“I got three gin and tonics for 15 dollars,” one, wearing a miniskirt and crop top, said to her two friends. “That’s not bad at all.”

They all compared their recent transactions. One had spent $15, the other $20.

“But the night is just starting,” said the taller friend, in skinny jeans. “I’m not buying my next drink.”

Outside at the restaurant, families continued to enjoy their quiet summer night at the shore. A toddler was in his high chair while a mother fed her baby from bottle. But at the bar downstairs, it might as well have been 2 a.m.

There was jumping up and down. Instagram and Snapchat were duly updated. Perfume, cologne, sweat and booze combined to create a special ambience.

“It smells disgusting,” said one young woman who had just arrived. “Is it always like this?”

“It’s fine,” her friend said, eyes rolling. “Let’s just get a drink.”

At 7:30 the band was playing, drinks were spilling, and a guy and girl danced slowly to a fast song. One of his hands was on her rear end, and the other was holding his Bud Light. He took turns locking lips with her and his beer bottle.

Marc Schilling stood close to the bar, looking at five blond women dance. In a group of adults barely out of college he looked older than his 46 years. At the Surf Lodge in Montauk, N.Y., he’d be buying a table and no one would blink an eye. Here, everything about him and his friends was out of place.

“I’ve been coming here for 25 years,” he said. “We come back and like to see these millennials enjoy it as much as we did back in the day.”

Mr. Schilling was leaving soon for home. He had to be up at 4 a.m. to go fishing.

One of the blondes saw him glancing.

“Hey, daddy,” she said, still dancing.

At 9 p.m., the basement stayed packed but dozens left because happy hour was over. The rest had just over two hours before the bar was to close and they’d make their way to Osprey Nightclub in nearby Manasquan.

“That’s where all the kids go,” said Mike Young, 38, earlier in the night. “What college kid goes home at 11:30?”

Lia Trangucci, 27, arrived just after happy hour ended, sober.

“I have a love-hate relationship with this place,” she said. “They do a great job at the raw bar and this place is just tradition. It’s been here for so long. But by 9 everyone is so drunk.”

The Parker House, Ms. Trangucci said, is where her parents come for dinner to enjoy the view.

“It’s this beautiful place, and then you go down to the basement and it’s fratty,” she said. “I’d rather not feel like I’m back in college. There’s no happy medium.”

But yet she comes back, again and again.

It was just after 11 p.m. and women swirled around to “Sweet Home Alabama” downstairs.

The basement continued to thump, but most people upstairs had left. The black and white tiles were filled with dirt and straws. Golf still played on the television. Employees started to sweep the floor and fill garbage cans.

At 11:30 in the basement, the lights turned on and no one flinched. People yelled, “one more song,” but the music had stopped. They stood around, waiting to leave or for something to happen.

The final stragglers made their way out rather less steadily than when they had first walked in. They gripped on more tightly to the railing as they made their way up to the ground floor. One female patron shed a few tears as she crawled into a taxi, and a guy sat down alone, waiting for his Uber.

The Parker House was done for the night, but the weekend was just beginning.

“Just wait until tomorrow,” said a taxi driver. “This place will be even crazier.”